Fix #110 Add instructions for dj-database-url and django-configurations in docs

[simonw]

It looks to me like this tip isn't purely specific to django-configurations - it should work if you only install dj-database-url without installing the rest of django-configurations.

I use dj-database-url on my Heroku projects without django-configurations so I like this pattern a lot.

[simonw]

So instead of headlining this session "Django Configurations", I suggest:

dj-database-url and django-configurations

Plus updated copy to reflect that using either package will enable this configuration pattern.

[pauloxnet]

You're totally right. I've updated the code to reflect your comment.

[simonw]

Thanks very much!